As disclosed in the following Patent Literature 1, an eccentric oscillation gear device that reduces the rotational frequency between two mating members at a predetermined reduction ratio is conventionally known. This eccentric oscillation gear device includes an outer cylinder which is fixed to one mating member, and a carrier which is fixed to the other mating member. The carrier rotates relative to the outer cylinder by the oscillating rotation of an oscillation gear mounted on an eccentric portion of a crank shaft. In the eccentric oscillation gear device, a rotational phase difference is generated in the torsional rigidity of a gear device, due to the eccentric direction of the assembled crank shaft (eccentric direction of the eccentric portion). This changes the phase of the torsional rigidity at reassembly, such as during maintenance. Therefore according to the invention disclosed in Patent Literature 1, a rotational phase adjusting unit, to determine the direction of the crank shaft, is disposed. Thereby even during reassembly after disassembly, the phase of the torsional rigidity becomes the same as the original state before disassembly.